Hair undergoes physical and chemical denaturalization of keratin proteins caused by various external factors and undergoes phenomena such as the generation of split ends or broken hair, the increase of frictional force of hair surface, the degradation of hair resilience and toughness, the mutual tangling of hair strands, hair dryness and the degradation of the feeling of the hair to the touch (see, Non-Patent Document 1).
For the purpose of preventing or overcoming such hair damage as described above, silicones, polysaccharides, polypeptides, polypeptide derivatives, surfactants, polyols, amino acids, fats and oils, plant extracts, ultraviolet absorbers or ultraviolet scatterers, and the like are used singly or as a combination of two or more thereof; among them, polypeptides or polypeptide derivatives attract attention from the viewpoints of affinity to the hair and effectiveness for the hair; accordingly a large number of technical proposals have been made.
For example, Patent Document 1 has proposed the use of a protein most abundantly containing amino acids having hydroxyl groups; Patent Document 2 has proposed the use of γ-polyglutamic acid; Patent Document 3 has proposed the use of an acylated peptide obtained by condensation reaction between a peptide obtained by hydrolyzing a protein and a lanolin fatty acid having a specific composition; and Patent Document 4 has proposed the use of a peptide in which side chain amino groups thereof are silylated.
The amino acid composition of each of these polypeptides or polypeptide derivatives is a mixture having a broad molecular weight distribution, however, or has an amino acid composition which is not necessarily constant, in such a way that the composition varies from one protein source to another protein source or from one production unit to another production unit; accordingly, the above-described effects tend to be varied. In addition, polypeptides or polypeptide derivatives having a large molecular weight are generally low in penetrability into hair, and modify the hair surface to effect only sensory improvements, involving properties such as appearance and feeling to the touch, and offer insufficient effects for substantively preventing or improving hair damage in the sense that the internal condition of the hair is improved. On the other hand, polypeptides or polypeptide derivatives having a small molecular weight have a low ability to remain on the hair, and accordingly have low persistence of the effect of the prevention or improvement of hair damage. Thus, in both cases, there have been problems that it is impossible to sufficiently prevent the generation of split ends or broken hair.    [Non-Patent Document 1] C. R. Robbins, “Chemical and Physical Behavior of Human Hair,” Chapter 8, Springer-Verlag, New York, 2002    [Patent Document 1] JP-A-2006-131579    [Patent Document 2] JP-A-2004-269430    [Patent Document 3] JP-A-11-302300    [Patent Document 4] JP-A-2000-086462